Liquid rocket engines operate by injecting liquid propellants into a combustion chamber where the propellants are ignited and combusted at high pressure and expelled to provide thrust. An injector is used to inject the liquid propellants, typically an oxidizer and a fuel, into the combustion chamber. The injector rapidly atomizes the liquid propellants into small droplets to promote efficient mixture and complete combustion.
Injectors for liquid rocket engines should be “stable,” or resistant to combustion instability. Combustion stability is discussed in detail in Chemical Propulsion Information Agency, “Combustion Stability Specifications and Verification Procedures,” CPIA Publication 655, January 1997. Combustion instability arises when pressure oscillations resonate in the combustion chamber and increase in magnitude until the chamber structurally fails. The oscillations grow due to positive feedback from the injector. In order to inject the oxidizer and fuel into the combustion chamber, the injector depends on a pressure difference between the fuel and oxidizer reservoirs (which could be tanks, pumps, or other vessels) and the combustion chamber. A higher pressure in the reservoirs causes the fuel and oxidizer to flow into the chamber. However, when pressure builds inside the combustion chamber, the pressure increase reduces the relative pressure difference across the injector (between the injector and the chamber), thereby reducing the flow of oxidizer and fuel into the chamber. When the fuel and oxidizer flow decreases, the pressure in the chamber drops. The resulting reduced pressure in the combustion chamber causes the opposite effect. The flow of fuel and oxidizer from the injector into the chamber increases, leading to another buildup of pressure. The flow of fuel and oxidizer thus contributes to the positive feedback of the pressure wave inside the combustion chamber, creating a resonant effect that can lead to catastrophic failure. The theory of combustion instability is described more fully in “Liquid Propellant Rocket Combustion Instability,” NASA SP-194, Washington D.C., 1972.
Thus, a stable injector is very desirable for a rocket engine. Several styles of injectors for liquid rocket engines are known in the art. Two examples are impinging jets and coaxial post injectors. An impinging jet injector incorporates alternating first and second sets of channels leading into the combustion chamber. The oxidizer is introduced through one set of channels, and the fuel through the other. The channels may be aimed at each other to direct the two propellants into each other as they enter the combustion chamber, to promote complete mixing. Impinging jet injectors require complicated and expensive machined parts to incorporate the various channels and jets and to keep the propellants separate until they are injected into the chamber. An example of an impinging jet injector is disclosed in U.S. Pat. No. 6,116,020 to Cornelius et al.
A coaxial post injector injects one propellant, typically but not necessarily the oxidizer, through a tube into the combustion chamber. The second propellant is injected through a channel surrounding the tube. Both propellants are thus injected around the same axis. The tubes in coaxial post injectors are relatively large, five or more millimeters in diameter, and they are rigid and stiff. The injector may include multiple tubes and channels spread across the injector face. An example of this type of prior art injector is shown in FIGS. 1a-1b. The prior art coaxial injector 100 includes an upper plate 112 housing a tube 116. The tube is attached to the upper plate 112 with a fastener 124 such as a screw, or by other methods of fastening such as brazing. The tube 116 extends down through the fuel plenum 126 into a channel 118 which is formed in the lower plate 114. The channel 118 is larger than the tube 116, so that an open annular region 119 is formed around the tube 116. The oxidizer 120 flows through the tube 116 into the combustion chamber 128. The fuel 122 in the fuel plenum 126 flows through the annular region 119 into the combustion chamber 128, where it mixes with the oxidizer 120. The two plates 112 and 114 may be machined, welded, or brazed together so that the tube 116 is centered in the channel 118, forming the annular region 119. The injector 100 requires complex centering features (for simplicity, not shown in FIGS. 1a-1b) in order to center the large, rigid tube inside the channel. When the tube is centered and the two plates are brazed together, the plates cannot be disassembled. The plates 112 and 114 are expensive to manufacture and maintain.
A typical micro-coaxial injector is disclosed in Samuel Stein, A High-Performance 250-Pound-Thrust Rocket Engine Utilizing Coaxial-Flow Injection of JP-4 Fuel and Liquid Oxygen, NASA TN D-126 (October 1959). An example of a prior art micro-coaxial injector is shown in FIGS. 2a-2b. The prior art micro-coaxial injector 200 employs injector tubes 216 with relatively longer lengths and smaller diameters as compared to the coaxial post injector. The injector tubes 216 have a higher length to outside diameter ratio than coaxial post injector tubes. This high ratio, approximately 6 or higher, provides less rigidity in the tubes, so that they are able to deflect elastically over a reasonable range of motion. The high ratio also reduces the need for the tubes to be exactly centered within the channels 218 through which the fuel 222 flows. In addition, the oxidizer flow through the tubes remains acceptably straight even when the tubes 216 are not aligned in the center of the channels 218. The micro-coaxial injector 200 thus reduces the need for complicated centering mechanisms.
A disadvantage of prior art micro-coaxial injectors is that the smaller elements require very tight manufacturing tolerances. As a result, these injectors can have problems controlling the distribution of the mass flow of the propellants to ensure good combustion stability and high efficiency. Therefore, there is a need for an improved micro-coaxial injector that can improve combustion stability and efficiency and reduce manufacturing costs.